Lee CSCE 312 TAMU 1 System Level I/O Instructor: Dr. Hyunyoung Lee Based on slides provided by Randy Bryant and Dave O’Hallaron

Lee CSCE 312 TAMU 2 Topics Unix I/O Robust reading and writing Reading file metadata Sharing files Standard I/O

Lee CSCE 312 TAMU 3 A Typical Hardware System main memory I/O bridge bus interface ALU register file CPU chip system busmemory bus disk controller graphics adapter USB controller mousekeyboardmonitor disk I/O bus Expansion slots for other devices such as network adapters.

Lee CSCE 312 TAMU 4 Reading a Disk Sector: Step 1 main memory ALU register file CPU chip disk controller graphics adapter USB controller mousekeyboardmonitor disk I/O bus bus interface CPU initiates a disk read by writing a command, logical block number, and destination memory address to a port (address) associated with disk controller. I/O bridge

Lee CSCE 312 TAMU 5 Reading a Disk Sector: Step 2 main memory ALU register file CPU chip disk controller graphics adapter USB controller mousekeyboardmonitor disk I/O bus bus interface Disk controller reads the sector and performs a direct memory access (DMA) transfer into main memory.

Lee CSCE 312 TAMU 6 Reading a Disk Sector: Step 3 main memory ALU register file CPU chip disk controller graphics adapter USB controller mousekeyboardmonitor disk I/O bus bus interface When the DMA transfer completes, the disk controller notifies the CPU with an interrupt (i.e., asserts a special “interrupt” pin on the CPU)

Lee CSCE 312 TAMU 7 Unix Files A Unix file is a sequence of m bytes:  B 0, B 1,...., B k,...., B m-1 All I/O devices are represented as files:  /dev/sda2 ( /usr disk partition)  /dev/tty2 (terminal) Even the kernel is represented as a file:  /dev/kmem (kernel memory image)  /proc (kernel data structures)

Lee CSCE 312 TAMU 8 Unix File Types Regular file  Binary or text file.  Unix does not know the difference! Directory file  A file that contains the names and locations of other files. Character special and block special files  Terminals (character special) and disks (block special) FIFO (named pipe)  A file type used for inter-process communication Socket  A file type used for network communication between processes

Lee CSCE 312 TAMU 9 Unix I/O The elegant mapping of files to devices allows kernel to export simple interface called Unix I/O. Key Unix idea: All input and output is handled in a consistent and uniform way. Basic Unix I/O operations (system calls):  Opening and closing files  open() and close()  Changing the current file position (seek)  lseek (not discussed)  Reading and writing a file  read() and write()

Lee CSCE 312 TAMU 10 Opening Files Opening a file informs the kernel that you are getting ready to access that file. Returns a small identifying integer file descriptor  fd == -1 indicates that an error occurred Each process created by a Unix shell begins life with three open files associated with a terminal:  0: standard input  1: standard output  2: standard error int fd; /* file descriptor */ if ((fd = open(“/etc/hosts”, O_RDONLY)) < 0) { perror(“open”); exit(1); }

Lee CSCE 312 TAMU 11 Closing Files Closing a file informs the kernel that you are finished accessing that file. Closing an already closed file is a recipe for disaster in threaded programs (Cf. Ch. 12) Moral: Always check return codes, even for seemingly benign functions such as close() int fd; /* file descriptor */ int retval; /* return value */ if ((retval = close(fd)) < 0) { perror(“close”); exit(1); }

Lee CSCE 312 TAMU 12 Reading Files Reading a file copies bytes from the current file position to memory, and then updates file position. Returns number of bytes read from file fd into buf  nbytes < 0 indicates that an error occurred.  short counts ( nbytes < sizeof(buf) ) are possible and are not errors! char buf[512]; int fd; /* file descriptor */ int nbytes; /* number of bytes read */ /* Open file fd... */ /* Then read up to 512 bytes from file fd */ if ((nbytes = read(fd, buf, sizeof(buf))) < 0) { perror(“read”); exit(1); }

Lee CSCE 312 TAMU 13 Writing Files Writing a file copies bytes from memory to the current file position, and then updates current file position. Returns number of bytes written from buf to file fd.  nbytes < 0 indicates that an error occurred.  As with reads, short counts are possible and are not errors! Transfers up to 512 bytes from address buf to file fd char buf[512]; int fd; /* file descriptor */ int nbytes; /* number of bytes read */ /* Open the file fd... */ /* Then write up to 512 bytes from buf to file fd */ if ((nbytes = write(fd, buf, sizeof(buf)) < 0) { perror(“write”); exit(1); }

Lee CSCE 312 TAMU 14 Unix I/O Example Copying standard input to standard output one byte at a time. Note the use of error handling wrappers for read and write. #include "csapp.h" int main(void) { char c; while(Read(STDIN_FILENO, &c, 1) != 0) Write(STDOUT_FILENO, &c, 1); exit(0); }

Lee CSCE 312 TAMU 15 Dealing with Short Counts Short counts can occur in these situations:  Encountering (end-of-file) EOF on reads.  Reading text lines from a terminal.  Reading and writing network sockets or Unix pipes. Short counts never occur in these situations:  Reading from disk files (except for EOF)  Writing to disk files. How should you deal with short counts in your code?  Use the RIO (Robust I/O) package from your textbook’s csapp.c file.

Lee CSCE 312 TAMU 16 The RIO Package RIO is a set of wrappers that provide efficient and robust I/O in applications such as network programs that are subject to short counts. RIO provides two different kinds of functions  Unbuffered input and output of binary data  rio_readn and rio_writen  Buffered input of binary data and text lines  rio_readlineb and rio_readnb  Cleans up some problems with Stevens’s readline and readn functions.  Unlike the Stevens routines, the buffered RIO routines are thread-safe and can be interleaved arbitrarily on the same descriptor. Download from csapp.cs.cmu.edu/public/ics/code/src/csapp.c csapp.cs.cmu.edu/public/ics/code/include/csapp.h

Lee CSCE 312 TAMU 17 Unbuffered RIO Input and Output Same interface as Unix read and write Especially useful for transferring data on network sockets  rio_readn returns short count only it encounters EOF.  rio_writen never returns a short count.  Calls to rio_readn and rio_writen can be interleaved arbitrarily on the same descriptor. #include “csapp.h” ssize_t rio_readn(int fd, void *usrbuf, size_t n); ssize_t rio_writen(nt fd, void *usrbuf, size_t n); Return: num. bytes transferred if OK, 0 on EOF ( rio_readn only), -1 on error

Lee CSCE 312 TAMU 18 Implementation of rio_readn /* * rio_readn - robustly read n bytes (unbuffered) */ ssize_t rio_readn(int fd, void *usrbuf, size_t n) { size_t nleft = n; ssize_t nread; char *bufp = usrbuf; while (nleft > 0) { if ((nread = read(fd, bufp, nleft)) < 0) { if (errno == EINTR) /* interrupted by sig handler return */ nread = 0; /* and call read() again */ else return -1; /* errno set by read() */ } else if (nread == 0) break; /* EOF */ nleft -= nread; bufp += nread; } return (n - nleft); /* return >= 0 */ }

Lee CSCE 312 TAMU 19 Buffered RIO Input Functions Efficiently read text lines and binary data from a file partially cached in an internal memory buffer  rio_readlineb reads a text line of up to maxlen bytes from file fd and stores the line in usrbuf.  Especially useful for reading text lines from network sockets.  rio_readnb reads up to n bytes from file fd.  Calls to rio_readlineb and rio_readnb can be interleaved arbitrarily on the same descriptor.  Warning: Don’t interleave with calls to rio_readn #include “csapp.h” void rio_readinitb(rio_t *rp, int fd); ssize_t rio_readlineb(rio_t *rp, void *usrbuf, size_t maxlen); ssize_t rio_readnb(rio_t *rp, void *usrbuf, size_t n); Return: num. bytes read if OK, 0 on EOF, -1 on error

Lee CSCE 312 TAMU 20 RIO Example Copying the lines of a text file from standard input to standard output. #include "csapp.h" int main(int argc, char **argv) { int n; rio_t rio; char buf[MAXLINE]; Rio_readinitb(&rio, STDIN_FILENO); while((n = Rio_readlineb(&rio, buf, MAXLINE)) != 0) Rio_writen(STDOUT_FILENO, buf, n); exit(0); }

Lee CSCE 312 TAMU 21 File Metadata Metadata is data about data, in this case file data. Maintained by kernel, accessed by users with the stat and fstat functions. /* Metadata returned by the stat and fstat functions */ struct stat { dev_t st_dev; /* device */ ino_t st_ino; /* inode */ mode_t st_mode; /* protection and file type */ nlink_t st_nlink; /* number of hard links */ uid_t st_uid; /* user ID of owner */ gid_t st_gid; /* group ID of owner */ dev_t st_rdev; /* device type (if inode device) */ off_t st_size; /* total size, in bytes */ unsigned long st_blksize; /* blocksize for filesystem I/O */ unsigned long st_blocks; /* number of blocks allocated */ time_t st_atime; /* time of last access */ time_t st_mtime; /* time of last modification */ time_t st_ctime; /* time of last change */ };

Lee CSCE 312 TAMU 22 Example of Accessing File Metadata /* statcheck.c - Querying and manipulating a file’s meta data */ #include "csapp.h" int main (int argc, char **argv) { struct stat stat; char *type, *readok; Stat(argv[1], &stat); if (S_ISREG(stat.st_mode)) /* file type*/ type = "regular"; else if (S_ISDIR(stat.st_mode)) type = "directory"; else type = "other"; if ((stat.st_mode & S_IRUSR)) /* OK to read?*/ readok = "yes"; else readok = "no"; printf("type: %s, read: %s\n", type, readok); exit(0); } bass>./statcheck statcheck.c type: regular, read: yes bass> chmod 000 statcheck.c bass>./statcheck statcheck.c type: regular, read: no

Lee CSCE 312 TAMU 23 How the Unix Kernel Represents Open Files Two descriptors referencing two distinct open disk files. Descriptor 1 (stdout) points to terminal, and descriptor 4 points to open disk file. fd 0 fd 1 fd 2 fd 3 fd 4 Descriptor table [one table per process] Open file table [shared by all processes] v-node table [shared by all processes] File pos refcnt=1... File pos refcnt=1... stderr stdout stdin File access... File size File type File access... File size File type File A (terminal) File B (disk) Info in stat struct

Lee CSCE 312 TAMU 24 File Sharing Two distinct descriptors sharing the same disk file through two distinct open file table entries  E.g., Calling open twice with the same filename argument fd 0 fd 1 fd 2 fd 3 fd 4 Descriptor table (one table per process) Open file table (shared by all processes) v-node table (shared by all processes) File pos refcnt=1... File pos refcnt=1... File access... File size File type File A File B

Lee CSCE 312 TAMU 25 How Processes Share Files A child process inherits its parent’s open files. Here is the situation immediately after a fork fd 0 fd 1 fd 2 fd 3 fd 4 Descriptor tables Open file table (shared by all processes) v-node table (shared by all processes) File pos refcnt=2... File pos refcnt=2... Parent's table fd 0 fd 1 fd 2 fd 3 fd 4 Child's table File access... File size File type File access... File size File type File A File B

Lee CSCE 312 TAMU 26 Standard I/O Functions The C standard library ( libc.a ) contains a collection of higher-level standard I/O functions  Documented in Appendix B of K&R. Examples of standard I/O functions:  Opening and closing files ( fopen and fclose )  Reading and writing bytes ( fread and fwrite )  Reading and writing text lines ( fgets and fputs )  Formatted reading and writing ( fscanf and fprintf )

Lee CSCE 312 TAMU 27 Standard I/O Streams Standard I/O models open files as streams  Abstraction for a file descriptor and a buffer in memory. C programs begin life with three open streams (defined in stdio.h )  stdin (standard input)  stdout (standard output)  stderr (standard error) #include extern FILE *stdin; /* standard input (descriptor 0) */ extern FILE *stdout; /* standard output (descriptor 1) */ extern FILE *stderr; /* standard error (descriptor 2) */ int main() { fprintf(stdout, “Hello, world\n”); }

Lee CSCE 312 TAMU 28 Standard I/O Buffering in Action You can see this buffering in action for yourself, using the always fascinating Unix strace program: linux> strace./hello execve("./hello", ["hello"], [/*... */]).... write(1, "hello\n", 6...) = 6... _exit(0) = ? #include int main() { printf("h"); printf("e"); printf("l"); printf("o"); printf("\n"); fflush(stdout); exit(0); }

Lee CSCE 312 TAMU 29 Unix I/O vs. Standard I/O vs. RIO Standard I/O and RIO are implemented using low-level Unix I/O. Which ones should you use in your programs? Unix I/O functions (accessed via system calls) Standard I/O functions C application program fopen fdopen fread fwrite fscanf fprintf sscanf sprintf fgets fputs fflush fseek fclose open read write lseek stat close rio_readn rio_writen rio_readinitb rio_readlineb rio_readnb RIO functions

Lee CSCE 312 TAMU 30 Pros and Cons of Unix I/O Pros  Unix I/O is the most general and lowest overhead form of I/O.  All other I/O packages are implemented using Unix I/O functions.  Unix I/O provides functions for accessing file metadata. Cons  Dealing with short counts is tricky and error prone.  Efficient reading of text lines requires some form of buffering, also tricky and error prone.  Both of these issues are addressed by the standard I/O and RIO packages.

Lee CSCE 312 TAMU 31 Pros and Cons of Standard I/O Pros:  Buffering increases efficiency by decreasing the number of read and write system calls.  Short counts are handled automatically. Cons:  Provides no function for accessing file metadata  Standard I/O is not appropriate for input and output on network sockets  There are poorly documented restrictions on streams that interact badly with restrictions on sockets:  Restriction 1: input function cannot follow output function without intervening call to fflush, fseek, fsetpos, or rewind. –Latter three functions all use lseek to change file position.  Restriction 2: output function cannot follow an input function with intervening call to fseek, fsetpos, or rewind.

Lee CSCE 312 TAMU 32 Pros and Cons of Standard I/O (cont.) Workaround for restriction 1:  Flush stream after every output. Workaround for restriction 2:  Open two streams on the same descriptor, one for reading and one for writing:  However, this requires you to close the same descriptor twice:  Creates a deadly race in concurrent threaded programs! FILE *fpin, *fpout; fpin = fdopen(sockfd, “r”); fpout = fdopen(sockfd, “w”); fclose(fpin); fclose(fpout);

Lee CSCE 312 TAMU 33 Choosing I/O Functions General rule: Use the highest-level I/O functions you can.  Many C programmers are able to do all of their work using the standard I/O functions. When to use standard I/O?  When working with disk or terminal files. When to use raw Unix I/O  When you need to fetch file metadata.  In rare cases when you need absolute highest performance. When to use RIO?  When you are reading and writing network sockets or pipes.  Never use standard I/O or raw Unix I/O on sockets or pipes.

Lee CSCE 312 TAMU 34 For Further Information The Unix bible:  W. Richard Stevens, Advanced Programming in the Unix Environment, Addison Wesley,  Somewhat dated, but still useful. Stevens produced authoritative works in:  Unix programming  TCP/IP (the protocol that makes the Internet work)  Unix network programming  Unix IPC programming